Films of thermoplastic resin include an unstretched film produced by ejecting and extruding a resin heated and melted in an extruder, through its T-die onto a casting roll followed by directly winding it up in coils; a monoaxially-stretched film produced by extruding a resin onto a casting roll and stretching it in the machine direction thereof; and a biaxially-stretched film produced by stretching a cast film both in the machine direction and in the cross direction thereof. All these films that are produced by ejecting and extruding a resin through a T-die onto a casting roll are trimmed by cutting off both their edges so as to have a constant thickness in the cross direction thereof, since both their edges are solidified thicker on the casting roll than the center part of the film owing to the characteristics of the highly-viscous resin melt. In case where films having the same resin composition are mass-produced, the resin of the cut and removed thick parts thereof may be recycled again as the starting material of films by heating and melting the parts in an extruder, and the resin may not be wasted; but in case where a variety of films that differ in their resin composition are produced each in a small amount, then the cut and removed thick parts of the produced films may be recycled as the starting material for the films only when the same films are again produced, and this is a bar to the increase in the yield of films.
For removing both edges of a resin film in the cross direction thereof, for example, a trimming method is proposed as in Patent Reference 1. The method comprises extruding a resin melt onto both surfaces of a metal sheet substrate to coat them, wherein the resin parts (edges) protruding from the metal sheet in the cross direction thereof are removed by nipping them with endless guide belts and tearing off them before the resin is cooled. Regarding the applicability of the method, the cut and removed parts may be applied to only limited use when they are recycled as the starting material of films, since various pigments and fillers are added to the resin. Therefore, in case where a variety of films are produced each in a small amount therein, the method is not expectable for the increase in the yield of films.
For reducing the economical loss of non-recyclable trimmed film wastes, a method is proposed in Patent Reference 2. The method relates to a film having an indispensable requirement of high quality, such as an electrically-insulating film of a biaxially-stretched polypropylene film for production of capacitors; and this comprises heating and melting a propylene polymer B in a first extruder, heating and melting a propylene polymer A in a second extruder, and co-extruding them through a flat sheet die, wherein the propylene polymer A is extruded by feeding it to both sides of the propylene polymer B, the resulting resin film is biaxially stretched, and then the propylene polymer A on both sides of the propylene polymer B are trimmed off. In the method, the propylene polymer B satisfying the indispensable requirement of high quality is used as possible effectively so as not to give trimmed film wastes. In the method, however, the propylene polymer B to be used must be so designed that it is compatible with the propylene polymer A in point of the molecular weight, the residual ash, the melt index and the melting point thereof, and therefore, the use of the method is limited and the method is not applicable to production of films of general-purpose thermoplastic resins.
Information of prior-art references relating to the present application includes the following:    Patent Reference 1: JP-A 2002-127099    Patent Reference 2: JP-A 08-336884